The miniaturization of latest electronic parts and consequent condensation of circuits composed of such cause the conducting elements of printed circuit boards to be more delicate, or to have higher density of interconnection.
Originally, printed circuit boards must have resistance to heat and chemicals, especially acids, to be durable in undergoing processes where intense heat or acids are employed, which fact becomes more significant for circuits comprising delicate conducting elements.
Reviewing the processes of producing copper conducting elements, at first a material copper foil is obtained via a rolling or electrolytic process. The electrolytic process, in which copper is electrodeposited onto a cathode from an electrolytic solution containing copper ions, is a popular method. Then the surface of the copper foil is roughened electrolytically to reinforce the adhesive strength thereof to a resin forming the substrate of a printed circuit board, and the processed surface is further coated with another metal, a copper alloy or a corrosion inhibiting agent so as to repress the interaction between the copper foil and the resin.
In this regard, Japanese Patent Publication No. 51-35711 discloses a copper foil which has a layer containing one material selected from the group consisting of zinc, indium or brass. Japanese Patent Publication No. 53-39376 discloses a copper foil which has two electrodeposited copper layers and another plating layer of a metal such as zinc, brass or nickel being inactive with respect to a resin to which the copper foil is laminated. Also, Japanese Patent Publication No. 58-500149 discloses a copper foil which has an electrodeposited zinc nodule or dendrite and a layer of a substance containing at least one selected from the group of copper, arsenic, bismuth, brass, bronze, nickel, cobalt and zinc.
However, it should be noted that when a copper foil having a layer of zinc or zinc alloy such as brass is used as the conductive element of a printed circuit board, the adhesion interface between the copper foil and the resin or its periphery is permitted to have lower resistivity to hydrochrolic acid. Precisely, since printed circuit boards must be immersed in a variety of acids or active solutions during the producing process thereof, the peel strength of the conducting elements of said conventional printed circuit boards, which easily corrode in acids, is deteriorated. Therefore, such conventional copper foils are easily peeled off the resins owing to thermal or mechanical shock when applied to the latest types of printed circuit boards, in which the width of a typical conductive element is very narrow.
Further considering other substances for the layer, nickel or tin, which are soluble in ferric chloride etching solution being popularly used, are insoluble in alkali etching solutions and have a fatal drawback of entailing stain which deteriorates the dielectric property of the printed circuit board.
Meanwhile, cobalt, which is soluble in both ferric chloride and alkali etching solutions, has lower resistivity to hydrochloric acid (not so extreme as zinc), which fact is a practical problem.